RELATIVISTIC EFFECTS IN SILICON CHEMISTRY - ARE THE EXPERIMENTAL HEATS OF FORMATION OF THE SILICON ATOM AND SIH4 COMPATIBLE

We have investigated the effects of relativity on the atomization ener gy of silane, SiH4, to attempt to resolve an earlier discrepancy betwe en theory and experiment. Using a spin-free no-pair Hamiltonian that i s based on a second-order Douglas-Kroll transformation, we find that r elativity reduces the atomization energy of SiH4 by 0.7 kcal mol(-1): a small change, but sufficient to bring theory and experiment into agr eement when we include experimental uncertainties. Excitation energies in the silicon atom, S-5(sp(3))-P-3(s(2)p(2)), and the atomic cation, P-4(sp(2))-P-2(s(2)p), which involve a reduction in the number of s-e lectrons, increase similar to 1.2 kcal mol(-1) when we include relativ ity. These excitation energies show an even larger increase, about 2.5 kcal mol(-1), when we include con correlation. By contrast, the ioniz ation potential, which involves no change in the number of s-electrons -electron configurations s(2)p(2) in the neutral atom and s(2)p in the cation-changes similar to 0.2 kcal mol(-1) when we include relativity . These predictions are consistent with the notion that s-electrons ar e the most affected by relativity, and that changes in the amount of s -character are related, qualitatively, to differential relativistic ef fects. (C) 1998 American Institute of Physics. [S0021-9606(98)02713-5] .